Polycrystalline silicon, commonly known as polysilicon, is used as a basic raw material for photovoltaic and semiconductor industries, and the demand for polysilicon is rapidly growing with recent development of the industries.
The method of manufacturing polysilicon is represented by a silicon deposition process (or chemical vapor deposition) which produces a solid phase of polysilicon from a raw material, silane gas.
According to the silicon deposition process, silicon fine particles are generated from the silane raw material gas through hydrogen reduction and thermal decomposition under a high-temperature circumstance, and the generated silicon fine particles are deposited in a form of polycrystalline on the surface of rods or particles. For example, a Siemens deposition method using a chemical vapor deposition reactor and a deposition method using a fluidized bed reactor are known.
In the silicon deposition process, there is a method of increasing an amount of raw material supply as one of methods of increasing a growth rate of polycrystalline silicon. However, it is not preferable that the raw material gas is excessively supplied, because a ratio of the raw material gas contributing to the deposition reaction is decreased to reduce a deposition amount (yield) of polycrystalline silicon.
Meanwhile, the silicon rod applied to the Siemens deposition method should maintain a surface temperature suitable for the deposition of polycrystalline silicon, and a temperature difference between the center and the surface of the rod increases due to convection, as the rod grows. Accordingly, when the rod grows to some extent and the temperature of the center portion of the rod reaches a melting point of polycrystalline silicon, meltdown is likely to easily occur. Thus, there is a problem in that the rod cannot be grown to have a large diameter of, for example, 150 mm or larger.
Therefore, many methods have been suggested in order to grow the rod diameter without meltdown of the rod. However, the reaction yield is still lowered, and there is an inconvenience such as modification of the reactor structure. Thus, there is a need to develop a technique of improving the drawbacks.